In the present invention, ultrasonic vibration is outputted to predetermined regions 60 of a first web 10A and a second web 20A from an output surface 332 of an ultrasonic horn 330 so that the predetermined regions 60 may be bonded. The output surface 332 includes a central output portion 332A in a center region 60C of three equal regions into which each predetermined region 60 is divided, and both-ends output portions 332B in both-ends regions 60S at sides of the center region 60C. A central maximum amplitude of the ultrasonic vibration outputted from the central output portion 332A is smaller than an end maximum amplitude of the ultrasonic vibration outputted from each of the both-ends output portions 332B. A central bonding strength of the center region 60C is smaller than an end bonding strength of each of the both-ends regions 60S.
